The present invention relates to electrolytic capacitors and the liquid electrolyte portion used in such capacitors.
For capacitors designed to operate at higher voltages (300 or more working volts), aluminum electrolytic capacitors have usually contained solutions of ethylene glycol with boric acid, ammonium borates and complexing agents for borates, such as mannitol. These electrolytes have the drawback of being thermally unstable. Particularly above 85.degree. C., these electrolytes tend to degrade. The mixture of ethylene glycol and acid anions may promote esterification and the formation of reaction products such as borate esters and water. As the solution ages, increasing amounts of borate esters and water have been observed. Higher concentrations could accelerate the attack on the anodic oxide film and cathode foil during electrically idle periods.
In order to passivate or protect the anodic oxide and cathode surfaces from attack during electrically idle periods, chromate, phosphate and similar anions are frequently added to electrolytic capacitors. The addition of chromates is highly undesirable because of their toxicity.
Another problem noted in high-voltage, high-capacitance devices is that the anodic oxide forming function of glycol-based electrolytes appears to decrease markedly in the presence of even minute quantities of chloride. It has been postulated that high leakage current areas or flaw sites may catalyze oxidation of ethylene glycol to corrosive species such as glyoxal and low molecular weight organic acids. These reaction products may attack imperfectly anodized portions of the device, such as the positive tab/terminal and the edges and ends of the positive foil.
In order to counter these problems, alternate materials have been employed as the solvent portion of the electrolyte, including dimethyl formamide, dimethyl acetamide and other substituted alkyl amides. However, these solvents have the further disadvantages of toxicity and volatility, and their use is generally restricted to capacitors rated below 300 volts.
Another problem with aluminum electrolytic capacitors is the susceptibility to acid attack of imperfectly anodized portions of the device, such as the positive tab/terminal and edges of the positive foil.
Another problem with high-voltage capacitors is their relatively short life, which has been attributed to the higher voltage accelerating chemical breakdown. Chemical breakdown is also accelerated by heat.
Over the years, several different formulations have been proposed for use in high-voltage electrolytic capacitors. For example, U.S. Pat. No. 4,373,176 to Finkelstein et al. discloses an electrolyte containing a tertiary amine or a dipropylamine mono salt of dodecanedioic acid in a solvent mixture of ethylene glycol, N-methyl-2-pyrrolidone and water.
U.S. Pat. No. 4,399,489 to Ross discloses an electrolyte for use in capacitors which includes ethylene glycol and N-methyl pyrrolidone and a solute mixture of diisopropylammonium pentaborate and dimethyl-ammonium or diisopropylammonium boro-dicatecholate.
U.S. Pat. No. 3,609,648 to Kihara et al. discloses a capacitor electrolytic solvent consisting of primarily ethylene glycol and a lesser amount of polyvinyl pyrrolidone.
U.S. Pat. No. 3,067,367 to Ross discloses the use in high-temperature spacer-less capacitors of a gel electrolyte which is comprised of polymers, such as polyvinyl pyridine, and plasticizers, such as ethylene glycol, which is present at a concentration of 5 to 70%.
What is needed is an electrolyte which is more resistant to thermal degradation and which does not attack the anodic oxide during electrification or cause degradation of device performance during periods of standing idle. The electrolyte system should also have a high sparking voltage. Capacitors containing such an electrolyte would exhibit a high degree of resistance to scintillation, corrosion and shorting of the electrodes.